The invention relates to a detector for the detection of electromagnetic radiation, including scintillators, a photosensor device and an intermediate layer. Detectors of this kind are used for the conversion of, for example X-rays into radiation in the visible light range.
Detectors for, for example, computed tomography are customarily constructed in combination with a photosensor device and scintillators, the photosensor device detecting the light emitted by the scintillator.
JP 09043356 A describes a detector in which a scintillator is arranged over a light detection panel. Between the scintillator and the light detection panel there is provided a transparent intermediate layer which allows unimpeded passage to the light converted from X-rays. This device utilizes slits which are cut into the scintillator, extend at right angles to the intermediate layer and are filled with an adhesive so as to attenuate crosstalk with neighboring channels by way of total reflection of the light on the slits, a channel being understood to mean a scintillator element with the associated photosensor. The crosstalk due to multiple reflection in the intermediate layer between photosensors and scintillator elements, however, cannot be avoided in this way.
A more essential factor governing the image quality of the CT detectors is the crosstalk between neighboring channels. Crosstalk between the neighboring channels is caused, for example by X-ray fluorescence quanta as well as by optical crosstalk. A detector detects the electromagnetic radiation with an appropriate resolution which is defined by the number of channels. In the case of a corresponding angle of incidence, the visible light rays can be reflected from one channel into a neighboring channel. This induces falsifications in the X-ray image to be formed, because radiation components of one channel are detected in another channel and give rise to an imaging value which does not correspond to reality in the subsequent image.
The crosstalk caused by X-ray fluorescence quanta can be prevented by providing suitable X-ray absorbers between the individual scintillator elements. Optical crosstalk, however, is caused by the light which is generated in the scintillator and is either transmitted directly into the neighboring scintillator element or reaches the neighboring photosensor by multiple reflection in the intermediate layer between the photosensors and the scintillator elements.
Optical crosstalk is prevented according to the present state of the art by fitting the absorber plates between neighboring scintillator elements in grooves which are etched so as to extend between the individual photosensors in the silicon wafer, so that the absorbers also divide the intermediate layer. However, this is possible only in the case of comparatively large linear detector structures.
Therefore, it is an object of the invention to provide a device in which the optical crosstalk due to multiple reflection in the intermediate layer is prevented.
This object is achieved in that at least parts of an intermediate layer contain substances absorbing electromagnetic radiation.
A scintillator consists of a plurality of scintillator elements or scintillator crystals. The photosensor device includes at least one photosensitive element or one photosensor for each scintillator element. The photosensor device detects the light emitted by the scintillator elements.
The crosstalk due to X-ray fluorescence quanta is prevented by the insertion of molybdenum plates or plates of another suitable metal, for example tantalum or lead, as absorbers between the scintillator elements. These absorbers also prevent direct optical crosstalk between the individual scintillator elements, but not indirect crosstalk in the intermediate layer.
The described method, where the absorbers also subdivide the intermediate layer by way of etched grooves in the silicon wafer, cannot be used for two-dimensional detectors. It is not possible to realize an exact two-dimensional metal grid structure and, moreover, it is not possible either to etch grooves into the silicon chip for small pixels of from 1 to 2 mm2 in CMOS chips, because the silicon surface must be available to photosensors and read-out structures.
The X-rays incident on the scintillator elements are converted into visible light rays and ideally reach, via an intermediate layer provided between the scintillator elements and the photosensor device, directly the photosensor arranged underneath the relevant scintillator element. Light quanta which do not reach the photosensor arranged underneath the scintillator element but are transmitted to a neighboring channel, for example, by multiple reflection on the transitional layers, falsify the X-ray image to be formed. The path traveled by such light components contributing to the crosstalk is substantially longer than the path traveled by the direct light rays to the photosensor arranged therebelow. Therefore, substances absorbing electromagnetic radiation are added to the intermediate layer, which substances prevent the light components contributing to the crosstalk from being transmitted into the neighboring channel during their longer travel.
The detector, used notably as an X-ray detector or as a CT detector, includes an intermediate layer which is preferably formed as an adhesive layer. A plurality of scintillator crystals or scintillator elements, subdivided by absorbers, are combined for the scintillator. These scintillator elements are connected to the photosensor layer arranged therebelow by means of the intermediate layer. The use of an adhesive enables a reliable coupling to be effectively realized between the scintillator elements and the relevant associated photosensors, thus enabling a high detector signal.
Carbon black is preferably mixed with the adhesive as the absorbing substance. The carbon black component should be selected to be such that the attenuation of the light during its passage along the direct path through the intermediate layer to the photosensor is comparatively low. The attenuation of the light traveling the longer path to a neighboring photosensor, however, is significantly higher.
The distribution of the absorbing substance may be limited to parts of the intermediate layer, be it that the manufacturing costs are thus increased. Preferably, a uniform distribution of the absorbing substance throughout the intermediate layer is realized.
The intermediate layer according to the invention, containing substances absorbing electromagnetic radiation, significantly reduces the cost of manufacture of a detector, because it is not necessary to use specially structured silicon chips and individual scintillator elements and absorber plates so as to manufacture a detector with a correspondingly improved crosstalk behavior. This simple, economical method of manufacture does not require a complex mask technique which would otherwise have to be used so as to provide structured black absorber layers between the individual scintillator elements and in the intermediate layer.
The object is also achieved by means of an adhesive which contains substances absorbing electromagnetic radiation and serves to connect scintillators and a photosensor device of a detector for the detection of electromagnetic radiation.
The invention also relates to a computer tomograph which includes an X-ray source for emitting an X-ray beam which is rotatable about a system axis, an arithmetic unit for calculating images of an object to be examined while utilizing the detector signals formed with different projections, as well as a detector which is struck by the X-ray beam and includes scinitillators, a photosensor device and an intermediate layer, substances absorbing electromagnetic radiation being present in at least parts of the intermediate layer.